n. In Roman antiquity, a body of infantry not corresponding exactly to either the regiment or the army-corps of modern times, composed of different numbers of men at different periods, from 3,000 under the kings to over 6,000 under Marius, usually combined with a considerable proportion of cavalry.n. In French history, one of numerous military bodies so called at different periods.n. Any distinct military force or organization comparable to the Roman legion.n. An extraordinary number; a great multitude.n. In zoology, a large group or series of animals, of indeterminate taxonomic rank, but generally of high grade.To enroll or form into a legion.